We plan to collect high-resolution data on a number of heme and iron-cluster metalloproteins and to determine the structural changes that occur upon change of oxidation state. The expected magnitude of these changes ranges from 0.3-0.05 E necessitating high resolution data to measure the changes with sufficient precision. We have shown in previous work that we can measure distances in Fe4S4 clusters with better than 0.01 E accuracy and have determined with the heme protein cytochrome c peroxidase that we can measure metal-ligand distances with an accuracy of 0.03 E (unpublished results). All of the proteins will be refined with SHELX and standard uncertainties on bond lengths determined with full-matrix analyses. Thus, this study will measure the small, but highly significant changes that occur in metallo centers upon reduction and confirm hypotheses about the rearrangements of electrons as reflected in changes in bond lengths and angles.